STEREO Eclipse

March
12, 2007: When scientists announce they're about
to calibrate their instruments, science writers normally put
away their pens. It's hard to write a good story about calibration.
This may be the exception:

On
Feb. 25, 2007, NASA scientists were calibrating some cameras
aboard the STEREO-B spacecraft and they pointed the instruments
at the sun. Here is what they saw:

"What
an extraordinary view," says Lika Guhathakurta, STEREO
Program Scientist at NASA headquarters. The fantastically-colored
star is our own sun as STEREO sees it in four wavelengths
of extreme ultraviolet light. The black disk is the Moon.
"We caught a lunar transit of the sun," she explains.

The
purpose of the experiment was to measure the 'dark current'
of STEREO-B's CCD detectors. The idea is familiar to amateur
astronomers: Point your telescope at something black and see
how much 'dark current' trickles out of the CCD. Later, when
real astrophotography is taking place, the dark current is
subtracted to improve the image.

In
this case, the Moon served as a black calibration disk backlit
by the sun. "The observation was no accident," she
says. Mission controllers arranged the alignment with a small
tweak to STEREO-B's orbit last December and engineers have been
waiting for the dark current data ever since.

"The
images have an alien quality," notes Guhathakurta. "It's
not just the strange colors of the sun. Look at the size of
the Moon; it's very odd." When we observe a lunar transit
from Earth, the Moon appears to be the same size as the sunâa
coincidence that produces intoxicatingly beautiful solar eclipses.
The silhouette STEREO-B saw, on the other hand, was only a
fraction of the sun's diameter. "It's like being in the
wrong solar system."

The
Moon seems small because of STEREO-B's location. The spacecraft
circles the sun in an Earth-like orbit, but it lags behind
Earth by one million miles. This means STEREO-B is 4.4 times
further from the Moon than we are, and so the Moon looks 4.4
times smaller.

Right:
STEREO A and B orbit the sun on either side of Earth. [animation]

STEREO-B
has a sister ship named STEREO-A. Both are on a mission to
study the sun. While STEREO-B lags behind Earth, STEREO-A
orbits one million miles ahead ("B" for behind,
"A" for ahead). The gap is deliberate: it allows
the two spacecraft to capture offset views of the sun. Researchers
can then combine the images to produce 3D stereo movies of
solar storms.

Of
particular interest are coronal mass ejections (CMEs), billion
ton clouds of electrified gas hurled into space by explosions
on the sun. "STEREO's ability to see these clouds in
3-dimensions will revolutionize our understanding of CMEs
and improve our ability to predict when they will hit Earth,"
she says.

The
STEREO mission is still in its early stages. The two spacecraft
were launched in Oct. 2006 and reached their stations on either
side of Earth in January 2007. Now it's time for check-out
and calibration. The first 3D views of solar storms are expected
in April.

So
science writers, ready your pens. If the calibration runs
are any indication, the actual data will be something to write
about.